Cleaning of polluted gas with a view to removing particulate or gaseous substances is an important and common process in today's industrialized society. A vast variety of techniques have been developed, and today there are often several methods to choose between when a gas cleaning plant is to be designed, even when very specific pollutants are to be removed.
Particulate pollutants are often removed by means of dynamic separators, such as cyclones, electrostatic precipitators or barrier filters, bag filters or cassette filters.
Gaseous pollutants are generally removed using an additive, either by absorption by the additive supplied in either dry or wet form, or by reacting gaseous pollutants with the additive supplied in either gaseous or liquid form, so as to obtain a particulate product. The reaction product is thereafter separated in a particle separator.
Cooling gas with a view to adapting the gas's temperature or to recovering heat therefrom, is also an important and common process. Heat transfer generally takes place either by means of heat exchangers of recuperative or regenerative type or by direct contact between the hot and the cold medium. Since this invention concerns heat transfer by direct contact between a gas and a liquid, other techniques will not be discussed.
One method advantageous in many respects consists of conducting a gas through a “rain” of finely divided liquid or past surfaces overflowed by a liquid. These methods make it possible to cool a hot gas as well as to capture particles in the liquid and to absorb or to react gaseous pollutant components of a polluted gas with the liquid. The liquid may also contain substances causing or promoting dissolved gaseous pollutant components to form solid particulates for easier separation thereof from the liquid.
The liquid is normally recycled in the contact device, but a portion thereof is removed, generally continuously, in order to use its heat in other applications and/or to be treated to separate pollutants therefrom. Thus the cooled and/or treated liquid can be recycled to the contact device to be used again.
Polluted gas washing plants can include contact devices or open towers where the polluted gas only encounters a finely divided liquid, and packed scrubbers or packed columns where the polluted gas flows through a tower filled with e.g. saddle-shaped or coil-shaped small parts, onto which liquid is sprayed so as to produce a liquid film which flows downwardly over essentially the entire total surface. However, since packed scrubbers do not fall within the field of the subject disclosure, such will not be discussed further herein.
Examples of contact devices or open towers, e.g. for separating sulphur dioxide from a polluted gas and/or for cooling of the gas in order to recover heat, are disclosed in U.S. Pat. No. 3,532,595. As such, U.S. Pat. No. 3,532,595 discloses both vertical towers and scrubbers with horizontal gas flow and liquid supplied at several levels or positions. U.S. Pat. No. 4,164,399, discloses a tower of less complex design, where liquid is supplied only at one level but is distributed after capture at several levels. U.S. Pat. No. 2,523,441, discloses a combination of an open tower with a packed section.
The above-noted techniques require that the liquid used in the contact device falls or flows downwardly by gravity. It is however also known to design contact devices or scrubbers which generate more or less horizontal liquid curtains through which the polluted gas flows. Two examples are disclosed in U.S. Pat. No. 2,589,956 and U.S. Pat. No. 3,691,731.
An intermediate design is disclosed in U.S. Pat. No. 4,583,999, wherein the washing liquid is supplied horizontally but, after some deceleration, descends as a rain of finely divided droplets.
Another gas contact device or tower is disclosed in DE-A1 33 41 318 or U.S. Pat. No. 3,532,595, wherein liquid is supplied at 4 to 6 levels. Each level has several nozzles to distribute small liquid droplets for gas and liquid contact. As such, each level is provided with nozzles arranged with a spacing of 0.5-1 meters (m), in a regular lattice. The distance between the levels is 1-2 m. The efficiency of the contact device or tower is largely dependent on the relative movement between the droplets and the gas. It is therefore generally preferred that the gas flows upwardly in a direction contrary to the descending liquid droplets, i.e. counter currently, but for various reasons there also exist gas contact devices or towers in which the gas descends in the same direction as the descending droplets, i.e. concurrently.
If it is desirable to increase the gas treatment efficiency using this method, it is necessary either to increase the height of the tower or to increase the flow of liquid. Whichever option is chosen, the consequence is increased pump work for a given volume of gas flow. Open gas contact devices or towers also suffer from the major disadvantage of requiring significant space. Significant space requirements also mean significant associated building costs since the towers typically must be relatively tall. As such, liquid to descend through the tower in the form of a rain of fine droplets must first be pumped up to a considerable height. Such pump work significantly increases operational costs associated with gas treatment.
Another open spray tower system is disclosed in U.S. Pat. No. 5,474,597. The open spray tower as disclosed uses nozzles arranged in a pattern whereby nozzles spraying liquid upwardly in the same direction as that of gas flow alternate with nozzles spraying liquid downwardly in the opposite direction as that of gas flow, for purposes of improving mass transfer. A disadvantage of the system is that the nozzles arranged to spray liquid upwardly plug when the system is not in operation and without a flow of liquid. When not in operation, accumulated slurry and particulates from nozzles arranged above spraying liquid downwardly caused plugging of unused nozzles, or worse yet, back flow into associated pumps.
Gas cleaning and gas cooling in wet-type contact devices or scrubbers, has for many decades been a well-established technique in process industries, power plants and incineration plants. Even with certain drawbacks, such as those noted above, this technique is well tried and must be considered both efficient and reliable. However, a significant drawback to such wet-type contact devices or scrubbers is spray nozzle clogging resulting in inefficient gas cleaning and gas cooling. The drawback of spray nozzle clogging needs to be addressed. As such, a method and/or apparatus that addresses costly nozzle clogging and the inefficient gas cleaning/cooling resulting therefrom is needed.